The invention pertains to a process for preparing nitrated pyridine-2,6-diamines in which a pyridine-2,6-diamine is contacted with a mixture of nitric acid and sulfuric acid. By the term "pyridine-2,6-diamines" are meant 2,6-diaminopyridine (DAP) and derivative compounds with substitution in the pyridine ring of either or both amino groups and/or carbon atom no. 4 (C-4).
The resulting 2,6-diamino-3,5-dinitropyridine (DADNP) is a suitable starting material for the manufacture of monomers for rigid rod polymers such as described in PCT Published Patent Application WO 94/25506 (reduction of the two nitro groups produces the desired tetra-amine). DADNP can also be used as an insensitive (safe) explosive and as a multifunctional organic reagent.
The nitration of DAP by having it react with a mixture of nitric acid and sulfuric acid is known from German Patent Publication No. 3,920,336. The drawback to this process is that it gives a DADNP yield of not more than 50% of theory.
Another disclosure of the nitration of DAP by reacting it with a mixture of sulfuric acid and nitric acid is Williams et al., J. Heterocyclic Chem., 8, 841-843 (1971). Williams discloses the use of concentrated sulfuric acid and a small amount of 90% nitric acid. Yield data other than for crude product are not given.
Further background art is Ritter et al., J. Heterocyclic Chem., 32, 585 (1995), which mentions the nitration of DAP using nitric acid and indicates the occurrence of by-product formation.
In view of the above it is desired to come up with a DAP nitration process which will produce a higher yield and so offer greater economic advantage as well as making for lower amounts of waste material.